1. Field of the Invention
The present invention relates to the field of copper plating systems and, more particularly, to a technique for replenishing copper in a plating solution.
2. Background of the Related Art
Plating systems, in which an object is immersed in a plating solution to plate metal onto the object, are well known in the art. A variety of metals can be plated by simple immersion or electroplated when electrodes are introduced in the solution. In copper plating, a plating solution such as a mixture of copper sulfate (CuSO.sub.4) and sulfuric acid (H.sub.2 SO.sub.4) is used as the source of copper to plate copper onto an object. Typically, a cathode is connected to the object that is to be plated (so that the object functions as the cathode electrode) and a potential is placed across the cathode and an anode. Copper ions in the solution will then be reduced onto the cathode electrode (namely, the object to be plated).
In the traditional copper plating approach, the anode electrode is usually made of copper which dissolves into the plating solution to replace the copper ions as the copper ions are depleted. However, for precision plating, inert anodes are utilized so that the anode does not change shape during the plating process. Instead of the copper ions being oxidized from the anode material, some other source of copper is needed. In this instance, copper containing material is introduced into the plating solution. That is, some external source is used to replenish copper ions in the solution as the copper ions are depleted from the solution due to the plating action.
A number of copper replenishing techniques are known in the art. See for example, U.S. Pat. Nos. 4,324,623; 5,516,414; and 5,609,747. However, the known replenishment techniques generally rely on the introduction of copper sourcing materials, such as CuSO.sub.4 and Cu(OH).sub.2, into the liquid bath. In some instances, an intermediary container (or bath) is utilized so that the copper sourcing material is not simply dumped into the solution.
Although this technique is adequate and acceptable for most general plating applications, it is not necessarily desirable where a very clean environment is desired. For example, in the fabrication of integrated circuits on semiconductor wafers (such as a silicon wafer), it is not desirable to have contaminant particulates in the cleanroom where the devices are manufactured. Since in many applications the copper sourcing material is in powder or granular form, the contamination factor is very high when these materials are present in the cleanroom. Similarly, any undissolved particles, from the addition of solid material into the solution, may have detrimental impact on the wafer being plated. Accordingly, it is appreciated that an improved approach to introduce copper sourcing material into a copper plating solution is desirable.